In the design of electrical or electronic circuitry, considerable saving of space can be realized by placing the electrical or electronic components directly on the printed circuit board, particularly by connecting the contacts of the components directly to printed conductors on the surface of the printed circuit board. In this case, it is not necessary to provide apertures in the printed circuit board to accommodate pin contacts projecting from the component.
Prior art contacts suffer from mechanical stresses at the soldering joints caused by the connection of the contacts to the printed circuit boards. During the soldering process, heat applied may result in bending of the printed circuit board so that mechanical stresses may result at the soldering joints after cooling. Due to the irregularities of the printed circuit boards such as roughness, the components to be soldered must be pressed against the printed circuit board to assure that all contact portions safely engage the conductors of the printed circuit board. After soldering, stresses are left in the external contact portions at the soldering joints.
Soldering is usually carried out at temperatures up to 250.degree. C. Due to different temperature expansion factors, the parts shrink differently which also results in stresses on the solder joints after cooling. Temperature variations also build up stresses at the soldering joints.
The mentioned problems occur particularly where a connector is provided with a large number of contact elements.
British patent application GB 2 173 652 and European patent application EP 0 200 449 teach to make the contact portions to be soldered flexible in a specific manner. This reduces the mechanical stresses at the soldering joints to some extent, however, the main problems mentioned above are not solved.
As will be explained in more detail, the present invention makes use of alloys having a so-called shape memory. Such alloys are generally described by "Legierungen mit Formgedachtnis", Vol. 259 Kontakt & Studium Werkstoff by Dieter Stockel, expert verlag. In such alloys, the martensitic texture transforms into an austenitic texture above a transformation temperature. A re-transformation into the martensitic texture takes place when the temperature goes below the transformation temperature, with a temperature hysteresis taking place; i.e., the re-transformation begins at a predetermined temperature below the transformation temperature. If such a shape memory alloy is trained in a suitable manner, the shape trained is attained again when the alloy is heated up to the transformation temperature.
In the mentioned book, also the application of such alloys to electrical contacts is described. A suitable shaped part of such an alloy co-operates with a contact element of resilient material. The interconnection between complementary contact elements is made when the part having the shape memory is in the martensitic phase and is deflected by the spring force of the contact element. In the austenitic phase, the spring force of the contact element is overcome by the shape memory alloy and thus establishes an intimate contact between the contact elements. Such a contact device eliminates the use of solder material. In such applications, the transformation temperature is reached at room temperature.
Japanese patent application (1984) 218757 discloses the use of shape memory material for the leads of semi-conductors. In this case, the shape memory serves to straighten the leads so that they can be threaded into holes of a printed circuit board.
From the Japanese patent application (1990) 30197, it is known to use a shape memory alloy for the leads of a component to be connected to a printed circuit board and to bend the leads after the insertion into the holes of the circuit board in order to retain the components in the holes through friction. If thereafter the leads are heated up to transformation temperature, e.g. during soldering, the leads straighten so that the solder may flow completely around the leads.
Japanese patent application (1990) 116151 discloses to combine leads of a shape memory alloy with an element of resilient material. The resilient material biases the lead away from the printed circuit board, and the memory alloy material of the lead deflects the element towards the printed circuit board above the transformation temperature.